Iron powder and sand filtration process for treatment of water contaminated with heavy metals and organic compounds

ABSTRACT

A water filtration device ( 3 ) and method which removes heavy metals and organic compounds from raw water is provided.

BACKGROUND OF THE INVENTION

Zero valent iron is an effective and economic reagent for removal ofheavy metals and for destruction of chlorinated organic compounds inwater because of its high reduction potential. It is known that zerovalent iron can be used to recover copper, silver and mercury in waterby electrochemical reduction or iron concentration (Case, O. P. 1974.In: Metallic Recovery from Waste Waters Utilizing Cementation.EPA-670/2-74-008, 9–23; Gold, J. P. et al. 1984. WPCF 56:280–286). Otherheavy metals such as lead, nickel, cadmium, chromium, arsenic, andselenium can also be removed from water using iron by reduction andprecipitation (U.S. Pat. Nos. 4,565,633; 4,405,464). Uranyl (UO₂ ⁺²) andpertechnetate (TcO₄ ⁻) can be effectively removed by iron throughreductive precipitation (Cantrell, K. J. et al. 1995. J. Hazard. Mater.42:201–212). Zero valent iron has also been used to remediatenitrate-contaminated water (Zawaideh, L. I. and T. C. Zhang. 1998. Wat.Sci. Tech. 38:107–115). Iron is also known to be effective fordechlorination of toxic organic compounds such as carbon tetrachlorideand trichloroethylene (Gilham, R. W. and S. F. O'Hannesin. 1994.Groundwater 32:985–967; Helland, B. R. et al. 1995. J. Hazard. Mater.41:205–216).

Zero valent iron in powder, granular, and fibrous forms can be used inbatch reactors, column filters, and permeable reactive barriersinstalled in groundwater aquifers for water treatment and metalsrecovery. However, iron particles in a filter rapidly fuse into a massdue to formation of iron oxides and deposition of the heavy metals. Thisfusion significantly reduces the hydraulic conductivity of the iron bed.To solve this problem, a mixture of iron and inert material such as sandhas been used in filter columns (Shokes, T. E. and G. Moller. 1999.Environ. Sci. Technol. 33:282–287). The mixed bed cannot be backwashedbecause iron and sand will be separated into different layers. Stirrers,rotating discs, and revolving drum reactors have been tested to keep theiron in motion in order to prevent the fusion of iron particles(Strickland, P. H. and F. Lawson. 1971. Proc. Aust. Inst. Min. Met.236:71–79; Fisher, W. 1986. Hydrometallurgy 16:55–67). However, themixing processes reduce the effectiveness of iron filters and increasewear of the reactors. A fluidized bed column has been developed toremove copper from highly acidic wastewater (U.S. Pat. No. 5,133,873).This process requires utilization of high flow rate and very fine ironpowder (i.e., 200 to 950 micrometers) in the filter.

Conventional treatment processes for removal of organic compounds andheavy metals from water are generally based on chemical precipitationand coagulation followed by conventional sand filtration (Dupont, A.1986. Lime Treatment of Liquid Waste Containing Heavy Metals,Radionuclides and Organics, 7th edition, Washington D.C., pp. 306–312;Eary, L. E. and D. Rai. 1988. Environ. Sci. Technol. 22:972–977; Cheng,R. C. et al. 1994. J. AWWA 86:79–90). Sand filtration alone is noteffective in removing heavy metals, especially arsenic and chromate,mainly because sand filter media have a low sorptive capacity for heavymetals. However, if the sand surface of the filter is coated with ironor aluminum hydroxide, the adsorption capacity of the filter media canbe significantly enhanced (Meng, X. G. 1993. Effect of Component OxideInteraction on the Adsorption Properties of Mixed Oxides, Ph.D. Thesis,Department of Civil and Environmental Engineering, Syracuse University,Syracuse, N.Y.).

In column studies, research has shown that cationic metals (Cu, Cd, Znand Pb) can be removed effectively by sand and granular activated carboncoated with ferric oxide (Benjamin, M. 1992. Metal Treatment atSuperfund Sites by Adsorptive Filtration, EPA/540/F-92/008; Jarog, D. etal. 1992. Adsorption and Filtration with Oxide-Coated Granular ActivatedCarbon, ACS Meeting, San Francisco, Calif., pp. 711–714; Edwards, M. andM. Benjamin. 1989. J. Water Pollut. Control Fed. 61:1523–1533). However,during these processes, sand and activated carbon have to be coatedperiodically prior to their placement in the filter. Further, theadsorptive capacity of the ferric oxide coating is much lower than thatof fresh ferric hydroxide precipitate.

Microfiltration (Martin, J. F. et al. 1991. J. Air Waste Manage. Assoc.41:1653–1657) and adsorption and magnetic filtration (Chen, W. Y. et al.1991. Res. J. Water Pollut. Control Fed. 63:958–964) have also beenstudied as means of removing heavy metals from water. Themicrofiltration process includes precipitation and filtration in twosteps. The main difference between this process and the traditionalprecipitation and filtration treatment is that the heavy metalprecipitates are removed directly through a membrane filter, eliminatingthe coagulation step. In the adsorption and magnetic filtration process,heavy metals are adsorbed onto fine magnetic particles coated withferrihydrite. The magnetic particles are then collected using a magneticfilter. Finally, the magnetic particles are regenerated by metaldesorption and then reused.

Dermatas and Meng (1996. Removal of Arsenic Down to Trace Levels byAdsorptive Filtration, 2nd Specialized Conference on Pretreatment ofIndustrial Wastewaters, Athens, Greece, pp. 191–198) tested anadsorptive filtration process for selective removal of arsenic fromwater. The process involved injection of ferric solution into the toplayer of the sand bed or within the sand filter. The stipulatedmechanism responsible for removal of arsenic is the coating of sandsurfaces with ferric precipitate and subsequent adsorption of arsenic. Adirect filtration process has been used for the treatment of sourcewater (G. P. Treweek, J. AWWA, February, 96–100 (1979); M. R. Collins,et al, J. Environ. Eng., 113(2), 330–344. (1987); J. R. Bratby, J. AWWA,December, 71–81 (1988)). The direct filtration process included additionof coagulants to the water followed by flocculation and filtration. Aflocculation time or hydraulic detention time of longer than 10 minuteswas needed which requires the installation of a large flocculationreactor prior to the sand filter.

A water filtration device has now been developed for removal of heavymetals and organic compounds, such as pesticides, from drinking water,waste water and soil washing solutions. The process for filtering watervia this device is based on use of a vibrating iron bed filter and asand filter.

SUMMARY OF THE INVENTION

The present invention is a water filtration device for removal of heavymetals and organic compounds from contaminated drinking water, wastewater and soil treatment solutions. The device comprises at least oneiron filter connected in series to a sand filter. Removal ofcontaminants from water is enhanced by the application of a source ofvibratory energy and/or an auger system to the iron filter. The augersystem may be used either in place of or in conjunction with thevibratory energy source. Oxidizers and coagulants may be mixed with thewater after passage through the iron filter and before passage throughthe sand filter to increase efficiency of contaminant removal fromwater. Also provided are methods for removal of heavy metals and organiccompounds from raw water using a water filtration device of the presentinvention.

Another object of the present invention is a water filtration device forremoval of heavy metals and organic compounds from contaminated waterthat employs on-line addition of iron solution with an in-line injectionport and a sand filter or multi-media filter, thereby eliminating theuse of an iron filter. This system can be used for treatment of watercontaining low levels of heavy metals. Iron solution is added upstreamof the sand filter to form a co-precipitate with the contaminants. Theco-precipitate is removed directly by the sand filter.

DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic of one embodiment of a water filtrationdevice of the present invention wherein the iron filter is subjected tovibration via an external source.

FIG. 2 depicts a process diagram for in-line injection. In this process,contaminated water enters the inflow, passes to an in-line mixer, mixingchamber and sand filter, and emerges as treated water.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a water filtration device which comprises acontinuously or intermittently vibrated iron bed filter or filters and asand filter. In this device, the fusion of iron particles in the filter,a drawback to previous iron filters, is prevented by continuous orintermittent vibration with an external or internal source of vibratoryenergy or by continuous mixing with an auger. Most of the waterpollutants are removed by the iron filter or filters. The residualpollutants in the iron filter effluent are further precipitated withferric ions generated by corrosion of iron particles and oxidation. Theprecipitates are removed directly by the sand filter. The iron filtercan also be eliminated from the system to form a direct co-precipitationfiltration process for the treatment of water containing low levels ofcontaminants.

In a preferred embodiment, the device of the present invention consistsof at least two filters in series, an iron filter 2 and a sand bedfilter 3, as shown in FIG. 1. A device which comprises two or more ironfilters and one or more sand filters in series can also be used. In oneembodiment, water for filtration is first collected in a container.Alternatively, the filtration device of the present invention can beplaced directly on line with a water supply source. The water to befiltered via the device of the present invention is contaminated withheavy metals and/or organic compounds that are harmful to human healthand is commonly referred to as “raw water”. Examples of heavy metals andorganic compounds which often contaminate the water include, but are notlimited to, arsenic, chromium, nickel, selenium, lead, cadmium, copper,PCBs, chlorinated organic compounds and pesticides. The raw water ispassed through the iron filter which comprises iron filings orparticles. The size of these particles can vary from fine powder tolarge granules and chips depending on the type of contaminant and waterflow rate. The chemical processes that take place within the iron filter(depending on the types of contaminants in the raw water) includereduction, precipitation, adsorption, dechlorination, and combinationsthereof. Since the incoming raw water generally contains dissolvedoxygen, up to saturation level of 8 mg/L, oxidation of the iron surfacetakes place immediately. Such oxidation fuses the iron particles into amonolithic structure, reducing the hydraulic permeability and renderingthe iron filter useless in a matter of hours or days. The fusion of ironparticles is therefore prevented in the instant invention by threedifferent methods involving applying a vibratory source 4 to the ironfilter 2.

In one embodiment, a source of vibratory energy 4 is applied to thegranular iron. The vibration is applied either continuously orintermittently. The vibrations keep the iron particle in motion and thusprevent the cementation of the matrix by oxide formation. The frequencyand the magnitude of the vibrations applied depends on the filter size,the type of granular iron used, and the types and concentration ofcontaminants present in the water, and can vary from low frequencies toultrasonic frequencies. The vibrations can be applied either externallyas depicted in FIG. 1 (i.e., applying vibration on the filter housingwith a commercially available vibrator) or internally directly on theiron powder with a vibratory probe. In order to increase thedistribution of vibration energy within the iron bed, a set of ribs orbaffles can be added to the vibratory probe. A combination of externallyand internally applied vibration can also be used.

In another embodiment of the present invention, applied vibration isused to regenerate the iron filter. The vibration frees the ironprecipitates which are formed during the oxidation process and the finerparticulates are carried by the water to the sand filter where they areretained and recovered during backwashing. In addition to vibration, anaugering system which is embedded in the iron and slowly circulates theiron can be used. The number and size of the augers required dependsupon the size of the filter, where augering, like vibration keeps theiron filter continuously regenerating.

In yet another embodiment, a combination of both vibration andcirculation via an auger is employed. In this embodiment, the core shaftof the auger consists of a vibratory probe and the combination ofvibration and circulation provides an improved separation of ironparticles within the filter.

During the filtration treatment, small of amounts of iron filings andgranular iron can be added into the vibrating iron filter, continuouslyor intermittently to maintain sufficient reactivity of the iron bed. Theiron bed can also be replaced partially or completely when itsreactivity decreases to a desirable level. Chemicals such as acids,bases, and oxidizing and reducing reagents such as ozone or hydrogenperoxide can be added to the water before it flows into the vibratingiron filter. The addition of the chemicals can control the reactivity ofthe iron bed and improve the removal and destruction of contaminants.

In yet another embodiment, on-line addition of iron solution and a sandfilter can be used for the treatment of water which contains only lowlevels, in the order of less than 1 ppm, of heavy metals. In thisembodiment, the iron solution is added to the water upstream of the sandfilter to form a co-precipitate with the contaminants. Theseco-precipitates are then removed directly by passage of the waterthrough the sand filter.

The sand filter 3 of the present invention is located downstream of theiron filter 2. The sand filter has a dual function in that itfacilitates precipitation of contaminants that are not removed in theiron filter, such as heavy metals, and it acts as a particulate filtercleaning the water from any produced solids. For example, small amountsof ferrous ions are released from the iron particles due to corrosion.Ferrous ions in the effluent coming out of the iron filter are oxidizedby dissolved oxygen and form ferric hydroxide precipitate. At the sametime, residual heavy metals in the effluent form co-precipitates withferric hydroxide rapidly and are removed by the sand filter. The processcontinues until the permeability of the sand filter is reduced. This isdetected by a pressure drop across the filter. The pressure isautomatically monitored and when it exceeds a specified value the sandfilter undergoes a backwash cycle by reversing the water flow as is donein conventional filter backwashing procedures. In one embodiment whereonly low levels of heavy metal contaminants are expected to be found inthe water to be treated, the iron filter 2 can be eliminated from thesystem to form a direct co-precipitation filtration process. The directcoprecipitation filtration process eliminates the iron filter in theiron powder-sand filter system (FIG. 1). Ferric solution and otherchemicals such oxidants and coagulants are added into the water pipe andother water distribution systems directly or through an on-line mixer,or through other device such as an injection port. The injection port islocated upstream of the sand filter 3 so that the chemicals can mix withthe water and convert the contaminants from soluble to particulate formbefore the water enters the sand filter bed 3. A conventional in-linemixer can be used to enhance the mixing process. Therefore, thisdistance between the injection port and the sand filter is a chamber formixing of the water and the iron solution in order to form aco-precipitate. Subsequently, the co-precipitate particles are removedby the sand filter. In FIG. 2, a process diagram for in-line injectionis provided.

A direct co-precipitation filtration process has lower costs and lesstotal area requirements for the treatment process. It eliminates theneed for flocculation reactor(s) that are usually required in the directfiltration process.

In some cases, additional oxidizers, such as potassium permanganate andchlorite salts, or coagulants, such as iron chloride and iron sulfate,are used to achieve complete removal of target contaminants from water.These coagulants and oxidizers can be added on-line by means such as ametering pump 5 placed between the iron filter 2 and sand filter 3 asshown in FIG. 1.

An arsenic filtration test was conducted by passing As-spiked tap waterthrough a vibrating iron filter packed with 400 grams of iron filings.Arsenic solutions containing 150 mg/L arsenic [100 mg/L As(V)+50 mg/LAs(III)] and 20 mg/L As(V) were passed through the iron filter. As(V)concentration was reduced from 20 mg/L in the influent to approximately0.3 mg/L in the filtered water (FIG. 2). When the influent arsenicconcentration was 150 mg/L, the average arsenic concentration in thefiltered water was 12 mg/L. The results indicated that if two ironfilters are used in series arsenic concentration can be reduced fromvery high concentrations to trace levels. The efficiency of the ironfilter can also be improved by increasing the retention time of water inthe iron bed.

The ability of the device of the present invention to processcontaminated water was demonstrates in water spiked with chromate ions(1000 μg/L of Cr (VI)). Cr(VI) concentration was reduced toapproximately 20 μg/L by the iron column. The sand filter furtherreduced Cr concentration to less than 3 μg/L. After 27 days oftreatment, the flow rate was increased from 0.34 gpm/ft² to 2.7 gpm/ft².The effluent Cr concentration increased slightly to approximately 5μg/L. Cr(VI) was effectively removed at a similar flow rate to theconventional sand filters.

The hydraulic retention time or the location of the iron injection portin the direct co-precipitation filtration process is determined by therate of co-precipitation of contaminants with ferric hydroxide. Theresults show that removal of As(V) and iron is a function of time ofco-precipitation. The mixed water was filtered through a 0.1 micronmembrane filter to remove the co-precipitate. As(V) concentration wasreduced from 50 μg/L in the influent to 3.3 μg/L within 1 minute ofmixing. At the same time, iron concentrations were reduced from 1000μg/L to 50 μg/L. Data showed that less than 10 minutes was required forthe removal of As(V) by co-precipitation with ferric hydroxide with thedevice of the present invention.

In addition, results showed that removal of As(V) by the directco-precipitation filtration process was an efficient means of watertreatment. Using water with an As(V) concentration of 50 μg/L, ferrichydroxide solution was added in-line at a concentration of 1.0 ppm,prior to passage of water through the sand filter. The residual arsenicconcentration in the filtered water was below 5 μg/L. During thefiltration process there was no arsenic breakthrough and the pressureacross the filter increased only gradually.

1. A water filtration apparatus, comprising at least one iron filterhaving a vessel and a bed of metallic iron particles within said vessel;at least one source of vibratory energy positioned so as to providevibratory energy to said bed of metallic iron particles without rocking,rotating or oscillating said vessel; and a sand filter positioneddownstream of said iron filter and hydraulically connected thereto. 2.The water filtration apparatus of claim 1, wherein said source ofvibratory energy is external to said bed of metallic iron particles. 3.The water filtration apparatus of claim 1, wherein said source ofvibratory energy is internal to said bed of metallic iron particles. 4.A water filtration apparatus, comprising at least one iron filter havinga vessel and a bed of metallic iron particles within said vessel; atleast one moveable auger which is in contact with said bed of metalliciron particles; and a sand filter positioned downstream of said ironfilter and hydraulically connected thereto.
 5. The water filtrationapparatus of claim 4, further comprising a source of vibratory energywhich is external to said bed of metallic iron particles.
 6. A methodfor removing heavy metals and organic compounds from a dilute aqueousstream using a filtration apparatus having at least one iron filtercontaining metallic iron particles, said method comprising the steps ofpassing the dilute aqueous stream through the water filtration apparatuswhile applying vibratory energy to said metallic iron particles, therebyremoving metals and organic compounds from the dilute aqueous stream soas to form an effluent; and passing the effluent through a sand filterpositioned downstream of said iron filter.
 7. A method for removingheavy metals and organic compounds from a dilute aqueous stream using afiltration apparatus having at least one iron filter containing metalliciron particles and at least one moveable auger inside of said at leastone iron filter, said method comprising the steps of passing the diluteaqueous stream through the water filtration apparatus while moving saidmovable auger so as to agitate at least some of said metallic ironparticles, thereby removing metals and organic compounds from the diluteaqueous stream so as to form an effluent; and passing the effluentthrough a sand filter positioned downstream of said iron filter.
 8. Themethod of claim 6, wherein the step of applying vibratory energy isperformed so that at least a portion of the metallic iron particles arevibrated in such a manner so as to scrub the surfaces thereof.
 9. Themethod of claim 6, wherein said step of applying vibratory energy isperformed intermittently.
 10. The method of claim 6, further comprisingthe step of adding a coagulant into the effluent between said steps ofpassing the dilute aqueous stream through the water filtration apparatusof iron filter and passing the effluent through a sand filter.
 11. Themethod of claim 6, wherein said vibratory energy is applied at afrequency in the range of about 20 Hertz to about 400 Hertz.
 12. Themethod of claim 11, wherein said vibratory energy is applied at afrequency in the range of about 50 Hertz to about 200 Hertz.
 13. Themethod of claim 12, wherein said vibratory energy is applied at afrequency of about 160 Hertz.
 14. The water filtration apparatus ofclaim 1, wherein said bed of metallic iron particles is a packed bed.15. The water filtration apparatus of claim 1, wherein the metallic ironparticles have a mesh size in the range of about 40 mesh to about 200mesh.
 16. The water filtration apparatus of claim 15, wherein themetallic iron particles have a mesh size in the range of about 80 meshto about 120 mesh.
 17. The water filtration apparatus of claim 4,further comprising a source of vibratory energy which is attached tosaid moveable auger.